Known today are photon-counting computed tomography (CT) devices using a photon-counting radiation detector (hereinafter, simply referred to as a detector). Unlike integrating detectors, photon-counting detectors output signals from which each of the X-ray photons having passed through a subject can be counted. In other words, a photon-counting detector outputs a pulse signal representative of a radioactive ray being incident on the detector, and with a waveform having a pulse height proportional to the radioactive energy. The detector can therefore estimate the radioactive energy incident on the detector by analyzing the pulse waveform. A photon counting CT device is therefore capable of reconstructing an X-ray CT image with a high signal-to-noise (SN) ratio.
The signals output from the photon-counting detector can also be used in measuring (separating) the energies of the X-ray photons. The photon counting CT device is therefore capable of visualizing a plurality of separate energy components from a piece of projection data collected from X-ray radiation at one tube voltage.
Required to enable the detector to distinguish subject materials from one another by measuring the energies of the X-ray photons having passed through the materials is calibration of the detector in a relation between detector outputs and energies of the fluorescent X-rays (incident energies) that are incident on the detector. In other words, when the detector is what is called an indirect conversion detector, detectors have variations in the characteristics of the silicon photomultiplier (SiPM) device (such as the multiplication factor and the operating temperature), and variations in the detection efficiency of scintillation light (variations in the detector geometric structure), and therefore, calibration of the detector outputs with respect to incident energies is required. The indirect conversion detector is a detector that converts incident X-ray photons into scintillation photons using a scintillator, amplifies the scintillation photons with the SiPM, and outputs the result.
In the energy calibration of photon-counting detectors, a standard source such as checking source or a radioisotope (RI) is often used. However, a checking source or an RI has limited radioactivity. When a detector is calibrated by placing a standard source with a radioactivity of several maga-becquerels to giga-becquerels and requiring strict management at a position approximately 1 meter away from the detector, a dose of radioactive rays being incident on each detecting element, which has a size of 1 millimeter by 1 millimeter, for example, is reduced prominently to a level of several becquerels to several-thousands becquerels. It is therefore infeasible to use a standard source in the energy calibration of the detector with several tens of thousands detecting elements.
A known approach for addressing issue is to obtain the K-absorption edge of a subject by positioning a subject for calibration between an X-ray tube and a detector, differentiating the energy profile of the linear attenuation coefficients of the subject, and estimating the local maximum.
With this approach in which the K-absorption edge of the subject is obtained by estimating the local maximum, however, a signal giving the local maximum in the differential waveform of linear attenuation coefficients, which is at the inflection point of the curve representing the K-absorption edge of the subject, shifts depending on the detector response. An indirect conversion detector using a scintillator, in particular, has a quite low energy resolution, that is, approximately 20 percent or so (estimated with 122 kilo-electron volts), so the detection of the exact K-absorption edge is quite difficult.
Furthermore, a cerium-doped lutetium yttrium orthosilicate (LYSO) crystal or LSO (Lu2SiO5) crystal, which includes lutetium (Lu) as a constituent element of the scintillator provided to the detector, include unstable Lu isotopes. When these unstable isotopes stabilize, three gamma rays (at 88 kilo-electron volts, 202 kilo-electron volts, and 307 kilo-electron volts) are emitted, while cascade transition takes place. An energy calibration method that measures the background radiation of Lu elements using this phenomenon is also known.
This approach, however, may require a long time and consume an enormous amount of power to calibrate, because Lu has low radioactivity. Furthermore, the calibration needs to be carried out during the time in which the device not in use, e.g., during night time. Furthermore, because radiation detecting apparatuses require energy calibration in a range from 0 kilo-electron volts to 150 kilo-electron volts, the precision of the energy calibration is reduced merely with three gamma rays.